How many planets are there in our Solar System? At last count, the number stands at eight, after little Pluto was reclassified as a dwarf planet and deprived of its status as part of the planetary system.


But that number may soon rise to nine – or even 10. That’s because astronomers have continued searching for evidence of additional planets. And evidence is mounting for the possibility of at least one more planet based on what appears to be its effects on the area around where it might be.

The Search Begins

In 2014, a team of astronomers – led by Scott Sheppard of the Carnegie Institution in Washington, DC, and Chad Trujillo of the Gemini Observatory in Hawaii – focused their attention on the Kuiper Belt, the region beyond Neptune composed mostly of icy rocks and other small objects that are called “trans-Neptunian objects” (TNOs).

Artist's impression of NASA’s New Horizons spacecraft encountering a Pluto-like object in the distant Kuiper Belt. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Steve Gribben

The team examined a dozen TNOs and discovered a strange similarity: All behaved virtually identically in their orbital trajectories. Their movements were not random (as would have been the case had they been under the gravitational influence of only Neptune and the Sun), leading the researchers to hypothesize that another body in space, one sufficiently large to qualify as a planet, must exist – but where exactly?

A Hypothesis Is Tested

When the results of the study were published, the report fell into the hands of two astronomers at CalTech: Mike Brown, who was notorious for his role in the demotion of Pluto to the status of dwarf planet, and Konstantin Batygin. They decided to test the article’s thesis with the goal of debunking the possibility of a new pretender to the throne of planetary status.

What they found, however, was quite the opposite of their hopes. Rather than undermining the hypothesis, their findings strengthened it! In early 2016, Brown and Batygin published their analysis of the orbital angle and movements of a total of six TNOs (a later paper boosted the number to 10) and their conclusion was even more compelling than the earlier team’s work.

The six most distant known objects in the Solar System with orbits exclusively beyond Neptune all line up in a single direction. Moreover, they are all tilted nearly identically away from the plane of the Solar System. The diagram was created using WorldWide Telescope. Image credit: Caltech/R. Hurt (IPAC)

The new analysis proposed a massive body (informally dubbed “Planet 9”) in a very distant, elliptical path around the Sun that causes all these objects in the Kuiper Belt to dip consistently in their orbits – around 30 degrees downward from the relatively flat orbital plane of the eight known planets.  The possibility of this orbital dip occurring randomly was calculated to be only 0.007 percent.

Brown and Batygin’s analysis also indicates that Planet 9’s unique orbit would completely set it apart from the eight known planets. The data shows that it would have what’s called an “anti-aligned orbit”; that is, its path around the Sun would be nearly perpendicular to the other planets, which would neatly explain the odd orbits of the TNOs they studied.

Theoretically, Planet 9 would be 10 times larger than Earth, have an average distance from the Sun 20 times greater than that of Neptune, and take around 15,000 years to make a complete solar orbit.


Artist’s Conception of a Kuiper Belt object. Image credit: NASA/JPL-Caltech/T. Pyle (SSC)

Not So Fast There!

Unsurprisingly, not all astronomers found the new discovery totally convincing. In particular, a team of Canadian researchers from the Canadian Astronomy Data Center and the University of Victoria, both in British Columbia, has theorized that the researchers’ paper may have fallen into the trap of “observational bias.”

Observational bias is a flaw in reasoning where researchers find a pattern simply because they are searching for one.

The Canadian team cites the relatively small original sample size of six objects. They suggest that examining more Kuiper Belt objects could easily upend Brown and Batygin’s hypothesis and, therefore, eliminate a so-called Planet 9 from the realm of possibility. (Brown and Batygin dispute this claim.)

Howzabout a Planet 10?

While the controversy about the purported existence of Planet 9 continues to roil, another team has already presented evidence that there might be a “Planet 10.” Using their own research and calculations, scientists at the University of Arizona recently theorized yet another previously unknown body that would qualify as a planet.

After its historic encounter with Pluto, the New Horizons spacecraft, depicted in the artist’s concept above, has a new destination: a small Kuiper Belt object known as 2014 MU69 that orbits nearly a billion miles beyond Pluto. Image credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

So maybe there are 10 planets, or maybe nine. Or, just maybe, we are left with our original total of a mere eight. Whatever the outcome, astronomers are actively engaged in searching for more planets to fill out the total. With the New Horizons spacecraft beginning to explore the Kuiper Belt, and with powerful new instruments like the James Webb Space Telescope and the land-based Large Synoptic Survey Telescope coming on-line in the next few years, astronomers may soon have the means to confirm visually the existence of the next named planet in the Solar System.